1. Field of the Invention
The present invention relates to a method of correcting an output of a NOx sensor, and more particularly to a method of correcting changes in the output of the NOx sensor that arise from output variations in individual NOx sensors and pressure fluctuations of a measurement gas, thereby ensuring a highly accurate sensor output.
2. Discussion of Related Art
There have been proposed various measuring methods and devices for detecting concentrations of NOx components, such as NO and NO2, contained in a measurement gas such as vehicular exhaust emissions. For instance, there is known a NOx sensor element configured to obtain the concentration of the NOx component in the measurement gas by reducing or decomposing the NOx component in the measurement gas and measuring an amount of oxygen generated by reduction or decomposition of the NOx component. Further, a NOx sensor equipped with such a sensor element is also known.
In JP-A-8-271476 (Patent Document 1), JP-A-11-237362 (Patent Document 2), and JP-A-2000-28576 (Patent Document 3), for instance, there is disclosed a sensor element for obtaining the concentration of the NOx component contained in the measurement gas. The sensor element has a laminar structure formed by laminating a plurality of solid electrolyte layers each having a suitable thickness integrally on each other. Within the laminar-structured element, there are provided a first internal space into which the measurement gas is introduced through a first diffusion controlling passage, a second internal space into which an atmosphere in the first internal space is introduced through a second diffusion controlling passage and in which the NOx component existing in the atmosphere is reduced or decomposed, main oxygen pumping means to control an oxygen partial pressure in the first internal space, measurement oxygen pumping means to pump out the oxygen in the second internal space, and current detecting means to detect a pumping current flowing upon a pumping action of the measurement oxygen pumping means. On the basis of a pumping current (a limiting current) detected by the current detecting means, an amount of the NOx component is detected.
The NOx sensor described above undesirably suffers from a problem that the NOx concentration obtained from an output of the NOx sensor (sensor output) does not coincide with an actual NOx concentration in the measurement gas in an instance where the pressure of the measurement gas fluctuates. More specifically explained, when the pressure of the measurement gas increases, the sensor output increases, so that the NOx concentration higher than the actual NOx concentration is detected. On the other hand, when the pressure of the measurement gas is lowered, the sensor output decreases, so that the sensor output to be obtained is lower than the actual NOx concentration in the measurement gas.
In the light of the above, an empirical formula is conventionally found on the basis of relationship of output changes of NOx sensor with respect to pressure of measurement gas (Bosch LSU 4 Information, LSU 4 part #0 258 006 066; Non-Patent Document 1). On the basis of such an empirical formula, the NOx concentration in the measurement gas under a reference pressure (generally an atmospheric pressure) is calculated.
In the meantime, the NOx sensor is mass-produced so as to have a construction similar to each other for use on automotive vehicles and the like. However, outputs from a large number of NOx sensors are not always identical with each other. That is, it is known that the outputs from the respective NOx sensors vary or fluctuate even under the same measurement condition due to subtle differences in the construction and the size among the NOx sensors. To eliminate such output variations among the individual NOx sensors, it is required to examine, for each of the individual NOx sensors, output changes relative to the pressure fluctuation of the measurement gas for obtaining the above-indicated empirical formula (correction formula) and to correct, based on the formula, an output value every time when the NOx concentration in the measurement gas is measured, for thereby obtaining the actual NOx concentration in the measurement gas.
However, it is quite troublesome and costly to examine, for each of a large number of produced NOx sensors, output changes with respect to the pressure of the measurement gas for obtaining the correction formula for each sensor and to correct the output value of each sensor when the NOx concentration in the measurement gas is actually measured. Thus, such an arrangement is not practically feasible.
Patent Document 1: JP-A-8-271476
Patent Document 2: JP-A-11-237362
Patent Document 3: JP-A-2000-28576
Non-Patent Document 1: home page;
Bosch LSU 4 Information, LSU 4 part #0 258 006 066